1------------- 2 MDEV Primer 3------------- 4 5For those of us who know how to use mdev, a primer might seem lame. For 6everyone else, mdev is a weird black box that they hear is awesome, but can't 7seem to get their head around how it works. Thus, a primer. 8 9----------- 10 Basic Use 11----------- 12 13Mdev has two primary uses: initial population and dynamic updates. Both 14require sysfs support in the kernel and have it mounted at /sys. For dynamic 15updates, you also need to have hotplugging enabled in your kernel. 16 17Here's a typical code snippet from the init script: 18[0] mount -t proc proc /proc 19[1] mount -t sysfs sysfs /sys 20[2] echo /sbin/mdev > /proc/sys/kernel/hotplug 21[3] mdev -s 22 23Alternatively, without procfs the above becomes: 24[1] mount -t sysfs sysfs /sys 25[2] sysctl -w kernel.hotplug=/sbin/mdev 26[3] mdev -s 27 28 29Of course, a more "full" setup would entail executing this before the previous 30code snippet: 31[4] mount -t tmpfs -o size=64k,mode=0755 tmpfs /dev 32[5] mkdir /dev/pts 33[6] mount -t devpts devpts /dev/pts 34 35The simple explanation here is that [1] you need to have /sys mounted before 36executing mdev. Then you [2] instruct the kernel to execute /sbin/mdev whenever 37a device is added or removed so that the device node can be created or 38destroyed. Then you [3] seed /dev with all the device nodes that were created 39while the system was booting. 40 41For the "full" setup, you want to [4] make sure /dev is a tmpfs filesystem 42(assuming you're running out of flash). Then you want to [5] create the 43/dev/pts mount point and finally [6] mount the devpts filesystem on it. 44 45------------- 46 MDEV Config (/etc/mdev.conf) 47------------- 48 49Mdev has an optional config file for controlling ownership/permissions of 50device nodes if your system needs something more than the default root/root 51660 permissions. 52 53The file has the format: 54 <device regex> <uid>:<gid> <permissions> 55 or @<maj[,min1[-min2]]> <uid>:<gid> <permissions> 56 57For example: 58 hd[a-z][0-9]* 0:3 660 59 60The config file parsing stops at the first matching line. If no line is 61matched, then the default of 0:0 660 is used. To set your own default, simply 62create your own total match like so: 63 .* 1:1 777 64 65You can rename/move device nodes by using the next optional field. 66 <device regex> <uid>:<gid> <permissions> [=path] 67So if you want to place the device node into a subdirectory, make sure the path 68has a trailing /. If you want to rename the device node, just place the name. 69 hda 0:3 660 =drives/ 70This will move "hda" into the drives/ subdirectory. 71 hdb 0:3 660 =cdrom 72This will rename "hdb" to "cdrom". 73 74Similarly, ">path" renames/moves the device but it also creates 75a direct symlink /dev/DEVNAME to the renamed/moved device. 76 77You can also prevent creation of device nodes with the 4th field as "!": 78 tty[a-z]. 0:0 660 ! 79 pty[a-z]. 0:0 660 ! 80 81If you also enable support for executing your own commands, then the file has 82the format: 83 <device regex> <uid>:<gid> <permissions> [=path] [@|$|*<command>] 84 or 85 <device regex> <uid>:<gid> <permissions> [>path] [@|$|*<command>] 86 or 87 <device regex> <uid>:<gid> <permissions> [!] [@|$|*<command>] 88 89For example: 90---8<--- 91# block devices 92([hs]d[a-z]) root:disk 660 >disk/%1/0 93([hs]d[a-z])([0-9]+) root:disk 660 >disk/%1/%2 94mmcblk([0-9]+) root:disk 660 >disk/mmc/%1/0 95mmcblk([0-9]+)p([0-9]+) root:disk 660 >disk/mmc/%1/%2 96# network devices 97(tun|tap) root:network 660 >net/%1 98---8<--- 99 100The special characters have the meaning: 101 @ Run after creating the device. 102 $ Run before removing the device. 103 * Run both after creating and before removing the device. 104 105The command is executed via the system() function (which means you're giving a 106command to the shell), so make sure you have a shell installed at /bin/sh. You 107should also keep in mind that the kernel executes hotplug helpers with stdin, 108stdout, and stderr connected to /dev/null. 109 110For your convenience, the shell env var $MDEV is set to the device name. So if 111the device "hdc" was matched, MDEV would be set to "hdc". 112 113---------- 114 FIRMWARE 115---------- 116 117Some kernel device drivers need to request firmware at runtime in order to 118properly initialize a device. Place all such firmware files into the 119/lib/firmware/ directory. At runtime, the kernel will invoke mdev with the 120filename of the firmware which mdev will load out of /lib/firmware/ and into 121the kernel via the sysfs interface. The exact filename is hardcoded in the 122kernel, so look there if you need to know how to name the file in userspace. 123 124------------ 125 SEQUENCING 126------------ 127 128Kernel does not serialize hotplug events. It increments SEQNUM environmental 129variable for each successive hotplug invocation. Normally, mdev doesn't care. 130This may reorder hotplug and hot-unplug events, with typical symptoms of 131device nodes sometimes not created as expected. 132 133However, if /dev/mdev.seq file is found, mdev will compare its 134contents with SEQNUM. It will retry up to two seconds, waiting for them 135to match. If they match exactly (not even trailing '\n' is allowed), 136or if two seconds pass, mdev runs as usual, then it rewrites /dev/mdev.seq 137with SEQNUM+1. 138 139IOW: this will serialize concurrent mdev invocations. 140 141If you want to activate this feature, execute "echo >/dev/mdev.seq" prior to 142setting mdev to be the hotplug handler. This writes single '\n' to the file. 143NB: mdev recognizes /dev/mdev.seq consisting of single '\n' character 144as a special case. IOW: this will not make your first hotplug event 145to stall for two seconds. 146